In our earlier filed international application published as WO 2009/036507, we described a radar imaging system for capturing an image of an object within an area of interest. The content of the international publication is incorporated by reference.
The radar imaging system described in WO 2009/036507 comprises a radar array with a plurality of transmitter elements and a plurality of receiver elements. The radar uses a coherent MIMO (multiple input multiple output) technique to generate synthetic elements. In simple terms, each transmitter transmits a uniquely coded signal which is received by the receivers. The codes of the coded signals are used to isolate each unique transmitter/receiver path so that a three-dimensional image can be constructed. Various coding schemes are discussed including frequency division multiplexing (FDM), orthogonal frequency division multiplexing (OFDM), step frequency sequences and versions of code division multiplexing (CDM).
Coherent MIMO radars offer a number of advantages, notably in terms of reduced cost compared with a fully-filled phased-array radar aperture, and the ability to simultaneously monitor all beams in a sector of interest in contrast to scanning beam radars.
In many respects MIMO radars have similar imaging characteristics to Synthetic Aperture Radar (SAR). This is because each receiver element of the MIMO radar separately decodes the reflections from each transmitter; likewise each receiver of a SAR separately collects the reflections from each transmitter position. The great advantage of this is it enables the phase shifts required to focus the transmitter on any one beam or pixel at any distance to be applied on reception. Hence, like SAR, a MIMO radar can perform imaging in the near field where the pixel size can be much smaller than the aperture size.
The imaging system and method described in our earlier patent application has proven useful for detecting concealed objects in, for example, walk-through body scanning but has not been able to adequately classify the object for risk assessment.
Reference may also be had to Published United States Patent Application 2008/0284636 assigned to Safe Zone Systems. This patent application also aims to detect objects using radar signals. The Safe Zone Systems approach is to transmit a radar signal of one polarization and to use complex neural network pattern recognition of the co-polar and cross-polar returns to identify if a weapon is present. The Safe Zone System does not create an image of the object, instead relying upon signatures in the radar signals to flag if a weapon or other threat may be present. Because Safe Zone Systems does not have any spatial resolution it cannot avoid false negatives and it cannot classify the threat. For instance, the Safe Zone Systems invention would identify a pair of metal frame glasses as a threat on the basis of the radar cross section because it cannot identify the location as being non-threatening. In essence, the Safe Zone Systems approach is to highlight as a risk any person that generates a radar cross section that is greater than the average radar cross section of a person without a weapon.
The reference to any prior art in this specification is not, and should not be taken as, an acknowledgement or any form of suggestion that the prior art forms part of the common general knowledge in Australia.